The present invention relates to printed circuit boards (PCBs). More particularly, the present invention relates to alterations in a PCB ground plane to remove resonance.
Printed circuit boards (PCBs) are widely used in the electronics and computer industries to mechanically and electrically couple individual components. A xe2x80x9cmotherboardxe2x80x9d in a personal computer (PC), used to mount, and connect, a central processing unit (CPU) with other associated component is but one common example of a PCB. Generally, a PCB comprises a number of layers through which electrical signals may be routed, separated by dielectric layers. The layers for routing electrically isolated, or the entire layer may be electrically conductive. Conductive layers may be used to efficiently provide access to a particular voltage level, or voltage plane, over the entire area of the PCB. PCBs with one or more power planes, at voltages, such as Vdd, and one or more ground planes are relatively common.
One design choice in routing signals within a PCB, is between routing on the top layer (microstrip routing) or routing in one of the inner layers (stripline routing). Microstrip routing typically provides faster signal speeds or xe2x80x9cflight timesxe2x80x9d at the expense of requiring more complete connections to the routing traces. Signal speeds of 153 ps/in for microstrip routing and 170 ps/in for stripline routing are typical. An advantage of stripline routing is that such a routing makes it easier to electrically isolate the signals using isolation lines and ground planes.
The design of a trace, or conductor for a particular signal path, depends on many factors. Two important factors being the locations of the points to be connected and the required impedance of the trace. The required impedance will typically be set by the components connected by the trace, with the actual impedance a function of the inductance and capacitance of the particular trace design. Eventually, the length, width, and geometry of the trace are defined for an acceptable signal routing scheme. However, there are combinations of clock speeds and signal values that degrade the quality of signal transmissions for a given signal routing scheme. Resonances within a PCB may occur when a signal on a single signal path, although isolated from other signals on the PCB, oscillates at, or near, an integer multiple of signal transit time. Such resonance may seriously degrade the performance of the PCB.
FIG. 1 depicts the stripline routing of traces 2 within a single layer of a PCB 4. The routing shown in FIG. 1 may be used within a RAMBUS RIMM module containing dynamic random access memory (DRAM) devices, RDRAMs, as licensed by Rambus, Inc. of Mountain View, Calif. The signal routing in FIG. 1 does not reflect an embodiment of the present invention. This signal routing may be susceptible to resonance under certain conditions.
Although the present invention is not intended to be limited to any particular PCB 4, or trace 2 design, an embodiment of the present invention may be used to improve signal transmission characteristics in a device such as a RDRAM. That is, under some combinations of clock speeds and signal values, resonance may decrease device performance and an embodiment of the present invention may be used to prevent, or reduce, the decrease in performance.
A method and apparatus for decreasing resonance in a printed circuit board (PCB) uses cuts in a ground plane to slow a signal passing through the ground plane. Cuts in the ground plane may be used alone or in conjunction with the lengthening of signal traces.